Download SBI3U5.2DihybridCrossWorksheet

TWO FACTOR CROSSES
Crosses that involve two traits, such as pod color and pod shape, are called two-factor
crosses. Predicting the outcome of two-factor crosses requires basically the same
procedure as that for crosses involving one trait. During meiosis, nonhomologous
chromosomes assort independently. This means that each of the chromosomes of any
pair of homologous chromosomes has an equal probability of ending up in a gamete with
either chromosome from any other pair of homologous chromosomes. The genes that are
located on nonhomologous chromosomes also assort independently.
Because of independent assortment, a plant that is heterozygous for two traits (genotype
AaBb) will produce equal numbers of four types of gametes - AB, Ab, aB, and ab.
In the example that follows, we will predict the results of a cross between two plants that
are heterozygous for both pod color and pod shape.
Sample Problem
What are the genotypic and phenotypic ratios in the offspring resulting from a cross
between two pea plants that are heterozygous for pod color and pod shape? What is the
phenotype of the parents in this cross?
Step 1 Choose letters to represent the genes in the cross.
Let’s use the letters we used in the one-factor crosses - G for green and g for yellow, N
for smooth, and n for constricted.
Step 2 Write the genotypes of the parents.
Since the parents are heterozygous for both traits, their genotype must be GgNn. The
cross can be
written as GgNn X GgNn.
Step 3 Determine the possible gametes that the parents can produce.
Each parent produces four types of gametes - GN, Gn, gN, and gn.
Step 4
Enter the possible gametes at the top and side of the Punnett square.
GN
Gn
gN
gn
GN
Gn
gN
gn
Step 5 Complete the Punnett square by writing the alleles from the gametes in
the appropriate boxes.
The alleles from the gamete above the box and the alleles from the gamete to the side of
the box are combined inside each of the boxes. Write the capital letter first for each pair
of alleles. The letters inside each box represent the probable genotypes of the offspring
resulting from the cross.
GN
Gn
gN
gn
GN
GGNN
GGNn
GgNn
GgNn
Gn
GGNn
GGnn
GgNn
Ggnn
gN
GgNN
GgNn
ggNN
ggNn
gn
GgNn
Ggnn
ggNn
ggnn
Step 6 Determine the phenotypes of the offspring.
In this example, 9/16 have green smooth pods, 3/16 have yellow smooth pods, 3/16 have
green constricted pods, and 1/16 have yellow constricted pods.
Step 7 Using the results of steps 5 and 6, answer the problem.
Note that in this example, as in many of the genetics problems you will encounter, you
are asked for more than just the ratios resulting from the cross. This is one reason why it
is important to read genetics problems carefully. In this example, the genotypic ratio is
1/16: 2/16: 1/16: 2/16: 4/16: 2/16: 1/16: 2/16: 1/16 = 1 : 2 : 1 : 2 : 4 : 2 : 1 : 2 : 1. The
phenotypic ratio is 9/16 : 3/16 : 3/16 : 1/16 = 9 : 3 : 3 : 1. The phenotype for the parent is
green smooth pods.
Practice Problems
In mice, the ability to run normally is a dominant trait. Mice with this trait are called
running mice (R_). The recessive trait causes mice to run in circles only. Mice with this
trait are called waltzing mice (rr). Hair color is also inherited in mice. Black hair (B_) is
dominant over brown hair (bb). For each of the following problems, draw a Punnett
square in the space provided and fill in the information on the indicated lines.
1. Cross a heterozygous running, heterozygous black mouse with a homozygous
running, homozygous black mouse.
Parental genotypes:
Phenotypic ratio:
2. Cross a homozygous running, homozygous black mouse with a heterozygous running,
brown mouse.
Parental genotypes:
Phenotypic ratio:
3. Cross a waltzing brown mouse with a waltzing brown mouse.
Parental genotypes:
Phenotypic ratio:
4. Cross a homozygous running, heterozygous black mouse with a waltzing brown
mouse.
Parental genotypes:
Phenotypic ratio:
5. Cross a heterozygous running, brown mouse with a heterozygous running,
homozygous black mouse.
Parental genotypes:
Phenotypic ratio:
6. Cross two heterozygous running, heterozygous black mice.
Parental genotypes:
Phenotypic ratio:
Genotypic ratio:
7. Cross a waltzing, homozygous black mouse with a heterozygous running, brown
mouse.
Parental genotypes:
Phenotypic ratio: